The present invention relates to a microbiological process for producing anthracycline anticancer antibiotics. More particularly, it relates to a process in which chemically simplified versions of intermediates on the identified biosynthetic pathway to daunomycin are bioconverted to active analogues of naturally occurring antibiotics.
Anthracyclines and, more notably, doxorubicin, daunorubicin, carminomycin and aclacinomycin have emerged as important chemotherapeutic agents in the treatment of a broad spectrum of human cancers. Understandably, their synthesis has attracted much attention.
Conventionally, anthracycline antibiotics are produced by aerobic fermentation of strains of Streptomyces. One of the drawbacks of conventional fermentations is that they are incapable of yielding analogues of the antibiotics when they are allowed to run their natural course. As a result, attempts have been made to develop more versatile syntheses for the antibiotics by chemical routes. Preferably, syntheses can be developed which can be readily modified to yield the anitbiotics and analogues which are more potent and less toxic.
The majority of the chemical routes which have been developed to date are only partially successful. Typically, they involve numerous reaction steps and they yield the compounds in only very small amounts. Several of the known synthetic approaches involve steps which are difficult to carry out chemically and provide intermediates in low yields. Many chemical routes break down at the glycosidation step. Of necessity any chemical synthesis must end by coupling the anthracycline aglycone with the amino sugar which constitutes the antibiotic. While in some of the literature this reaction is reported to occur in a rather high percent yield, in actuality the laboratory procedures which have been developed provide only a few milligrams of product.
Thus, although the chemical structure of these antibiotics have been known for some time, there is still a need for a versatile and effective synthesis of these antibiotics and, more importantly, a synthesis capable of yielding desired product in large amounts that is suitable for industrial application.
Recently, studies have been directed to the biosynthetic pathway to daunomycin in various Streptomyces. While some portions of the pathway remain unidentified, it is now apparent that aklavinone and .epsilon.-rhodomycinone are on the pathway. Yoshimoto et al, The Journal Of Antibiotics, Vol. 33, No. 10, Oct. 1980, pp. 1158-65, proposes a pathway which proceeds via a hypothetical decaketide through aklavinone to .epsilon.-rhodomycinone and from there via decarbomethoxylation of the C-10 position to daunomycin. There is also evidence that carminomycin is reached in the pathway.
The identified pathway is revealed as having the following enzymic capacities:
11--hydroxylation PA1 10--decarbomethoxylation PA1 4--methylation PA1 7--glycosylation PA1 7--reduction PA1 13--hydroxylation and ketonization. PA1 (ii) R.sup.8 =OH,R.sup.9 =H PA1 (iii) R.sup.9 OH, R.sup.8 =H PA1 (iv) R.sup.8 =R.sup.9 =OH.
While these pathways and enzymic capabilities are evident in the literature, with limited exceptions, prior efforts have concentrated on the pathway itself and the production of natural occurring products. Its ability to produce structural analogous of the natural products has not been defined.